Chapter 9: Microbial Growth

Generation time

The time it takes for the population to double through one round of binary fission.

Microbial Growth

a. Increase in cell size

b. Increase in population size

Binary Fission

Bacteria normally reproduce by _____ _____ (asexual reproduction)

Exponential Growth

Each new fission cycle increases the population by a factor of 2.

N_n=(N₀)2ⁿ

Equation for calculating population size over time.

N_n

The number of cells at any generation n

N₀

The initial number of cells

n

The number of generations (# divisions)

Microbial Growth Curve

____ ____ of a bacterial culture is represented by the logarithm of the number of live cells plotted as a function over time.

1. Lag Phase

2. Log Phase

3. Stationary Phase

4. Death or decline phase

Lag Phase (1)

No increase in number of living bacterial cells

Log Phase (2)

Exponential increase in number of living bacterial cells.

Stationary Phase (3)

Plateau in number of living bacterial cells; rate of cell division and death roughly equal.

Death or decline phase (4)

Exponential decrease in number of living bacterial cells.

Chemostat

A culture vessel fitted with an opening to add nutrients (feed) and an outlet to remove contents (effluent), effectively diluting toxic wastes and dead cells.

• Maintaining volume so it doesn’t overflow

Direct (total) cell count

• Counting chambers

  • Fluorescence staining techniques

• Electronic counters

Don’t know what cells are dead or dying

• Coulter counter

• Petroff-Hausser Chamber

Direct (Viable) cell count

• Plating methods

  • Spread plate

  • Pour

(Start with a serial dilution of the culture)

• Membrane filtration method.

Serial Dilution

involves diluting a fixed volume of cells mixed with diluting solution using the previous dilution as an inoculum. The result is the dilution of the original culture by an exponentially growing factor.

Pour plate method

sample is mixed in liquid warm agar (45-50 C) poured into a sterile Petri Dish and further mixed by swirling. This process is repeated for each serial dilution prepared. The resulting colonies are counted and provide an estimate of the number of cells in the original volume sampled.

Spread Plate Method

Poured onto solid agar and then spread using a sterile spreader. This process is repeated for each serial dilution prepared. The resulting colonies are counted and provide an estimate of the number of cells in the original volume samples.

Indirect cell counts

• Turbidity/Cloudiness

• Metabolic activity

• Dry weight

fragmentation and Budding

Alternative patterns of cell division

Environmental factors that influence microbes

• Oxygen requirements

• Temperature

• pH

• Osmotic pressure

• Barometric pressure

Aerobes

utilize oxygen (and can detoxify toxic products)

Anaerobes

do not utilize oxygen

Obligate aerobes

organism that requires oxygen for growth

Obligate Anaerobes

Can grow w/o oxygen

Facultative Anaerobes

Utilize oxygen but can also grow w/o it.

Aerotolerant Anaerobes

Do not utilize oxygen but can survive and grow in its presence.

Microaerophiles

Requires a small amount of oxygen.

Superoxide Dismutase and Catalase

Most cells have developed enzymes that neutralize these chemicals.

Capnophiles

organism that requires carbon dioxide levels higher than atmospheric concentration.

0.04%

Atmospheric CO₂

Chemically defined media

Exact chemical composition is known

Complex media

Precise chemical composition of the medium is not known

Selective media

Suppress unwanted microbes and encourage desired microbes. e.g., Blood agar, McConkey agar, Mannitol Salt agar (MSA), Eosin methylene blue agar (EMB)

Differential Media

Make it easy to distinguish colonies of different microbes. e.g., McConkey agar, MSA, Eosin methylene blue agar (EMB)

Enriched media

Encourages growth of desired microbes. e.g., blood agar, chocolate agar.

Barophiles

Can survive under extreme pressure and will rupture if exposed to normal atmospheric pressure.

• Barometric Pressure

Hypotonic

an environment in which the solute concentration inside a cell exceeds that outside the cell, causing water molecules to move into the cell, possibly leading to swelling and possibly lysis.

Isotonic

a solution in which the solute concentrations inside and outside the cell are approximately equal, thereby creating no net movement of water molecules across the cell membrane.

Halophiles

Require a high concentration of salt (hypertonic condition)

Osmotolerant

Does not require a high concentration of solute but can tolerate it when it occurs.

Minimum, maximum, optimum.

Organisms exhibit 3 cardinal temperatures.

Neutrophiles

organism that grows best at a near a neutral pH of 6.5–7.5

Acidophiles

organism that grows optimally at a pH near 3.0

Alkaliphiles

organism that grows optimally at pH above 9.0

Psychrophiles

Grows at 0 Celsius and below; optimum temp <15 degree Celsius.

Mesophiles

Optimum temperature 20-45 degrees celsius.

• Human microbes

Thermophiles

Optimum temp of 50 degrees celsius to maximum of 80 degrees celsius

Hyperthermophiles

Ranges from 80 degrees celsius to a maximum of 110 degrees celsius.